Amino Acids

Amino acids are the building blocks of proteins. The 20 naturally occurring amino acids are linked together in different sequences to generate the many forms of proteins that exist.

The arrangement and chemistry of the constituent amino acids are fundamentally important in determining the shape and function of the resulting protein.

For all but one of the naturally occurring amino acids, a central carbon atom is chemically bonded to a hydrogen atom (H), amine group (NH2), and a carboxylic acid group (COOH). The remaining bonding site of the carbon atom is occupied by various chemical groups, which are generically termed R groups.

The R group varies according to the particular amino acid. It is this variable chemistry that is key in dictating the contribution of the particular amino acid to the protein's shape and function.

An R group can be simple. For example, the R group of alanine is simply a hydrogen atom. However, R groups may be far more complex. One example is phenylalanine, which consists of an aromatic ring attached to CH2.

Other R groups contain sulfur atoms, or various arrangements of amine, carboxylic acid, and/or a carbon to which an oxygen atom is double-bonded.

The differing R group chemistries bestow varying degrees of charge to the amino acid, which in turn determines the tendency of the amino acid to associate with water. A ring structure, such as that present in phenyalanine, is more hydrophobic ("water-hating") than R groups such as hydrogen. An amino acid containing a more hydrophobic group will tend to avoid water. In an intact protein, such amino acids will tend to be associated with pockets or recesses in the protein's three-dimensional structure.

The exception to the common amino acid arrangement is proline, which contains a five-carbon, pentagon-shaped ring with an associated amide and a carboxylic acid.

Amino acids form bonds with one another to generate a chain known as a peptide. Peptide chains then bond together to generate the protein.

The tendency of the constituent amino acids to associate with water or avoid water drives the protein to adopt a three-dimensional shape that is the most thermodynamically stable. Thus, proteins can be more spherical, with the hydrophobic residues buried, or can assume a flatter shape reminiscent of a pleated (zigzag) piece of paper.

The various protein configurations are important to the overall function of the protein. For example, proteins that catalyze chemical reactions (enzymes) can contain a site that can be occupied by a portion of another protein. This "lock and key" fit is crucial in allowing the enzyme catalyzed reaction (such as the removal or addition of chemical groups) to take place.

Amino acids are important in sports precisely because they constitute proteins. Growth or repair of muscles requires the availability of protein. Supplementation of the diet with amino acid-enhanced nutrients is used to increase available amino acids used to build muscle mass. Because there are risks to nutrition alteration, all nutrient enrichment programs, especially those having controversial side effects, should be taken only after consultation with a physician and with the approval of a sports governing body.

SEE ALSO Amino acid supplements; Creatine supplementation; Dietary supplements; Glutamine supplementation; Malnutrition.